1. Field of the Invention
The present invention relates to a proximity sensor and proximity sensing method for detecting proximity of an object to be sensed based on a variation in capcitance between a sense electrode and ground.
2. Description of the Related Art
As known in the art, there have been sensors for sensing proximity of an object. The sensors of such the type can sense proximity of the object by detecting frequency or duty ratio which vary in response to the capacitance between a sensor portion or sense electrode and ground (see, for example, JP 2002-14174A, paragraphs 0082-0089, FIGS. 8 and 9). Such proximity sensors have a problem, however, because the detected value is easily influenced by ambient environmental factors such as temperatures, humidity, and extraneous noise. Therefore, the above-described sensor is provided with a thermistor or a semiconductor temperature sensor that exhibits a certain temperature characteristic for compensation of the temperature dependence of an analog circuit.
In addition to the increased cost of adding a thermistor or a semiconductor temperature sensor, the above-described sensor has another problem because it is difficult to match the temperature characteristic of the thermistor or semiconductor temperature sensor with temperature characteristics of the analog circuit. In addition, the above-described sensor which detects the variation in capacitance of the sense electrode provides a variable component, which is superimposed on an initial capacitance of the sense electrode or the analog circuit. Further, the value of the sensed output voltage from the sense circuit is dependent on the supply voltage and is finite. Therefore, if the output value has an upper limit equal to the supply voltage, it is only possible to use the value of the voltage range that remains after subtracting the value of the initial capacity from the supply voltage. Thus, the dynamic range can not be made large enough to perform high-precision detection.